Synthetic silica either in the form of silica aerogel or as a precipitated silica is used as a constituent of thermal insulating materials.
Thus German AS No. 2036124 (acknowledging British application No. 28052, filed June 10, 1970) describes an insulating sheet which consists of a permeable jacket of glass fiber fabric or cotton and silica aerogel which is mixed with a turbidity agent such as titanium dioxide.
The German AS No. 1671186 (acknowledging British application No. 43668, filed Sept. 29, 1966) describes a process for the production of a thermal insulating material consisting of an aerogel based on silica, aluminum silicate and a turbidity agent.
These known thermal insulating mixtures have the disadvantage that their use is limited by the temperature instability of the SiO.sub.2 components. Thus the specific surface area, for example, of a pyrogenic silica steadily decreases with increasing temperature beginning with 550.degree. C. and the silica particle size decreases at about 950.degree. C. through which the desired thermal insulation capacity is drastically decreased.